Metal anode for electrochemical processes

ABSTRACT

An anode for electrochemical processes comprises a basis metal and a new electrochemical active substance for the cover layer, which substance is thallium palladate having the formula TlPd3O4.

The present application is a continuation-in-part of our copendingapplication, Serial No. 516,996 filed October 22, 1974 now U.S. Pat. No.3,991,158.

BACKGROUND OF THE INVENTION

The present invention relates to a metal anode for electrochemicalprocesses.

Certain patents and patent applications, e.g. German Democratic RepublicPatent 55,323, German Democratic Republic Patent No. 77,963, GermanPublished application No. 1,671,422, German Published application No.1,917,040, German Published application No. 1,813,944, German Publishedapplication No. 1,962,860, and German Published application No.2,200,500 disclose metal anodes containing cover layers of the mostvarying composition in order to eliminate the drawbacks of anode basismetals, particularly where the basis metal is a valve metal, such astitanium, tantalum, zirconium, niobium, etc.

However, most of the substances suggested for application as a coverlayer, or as an essential component thereof, have unfortunately turnedout to be disadvantageous because (a) their electrical conductivityand/or their chemical and electrochemical stability is not satisfactoryor (b) a number of the suggested substances cannot at all or cannot veryeasily be applied to the anode carrier member without adverselyaffecting the requirements made respecting a cover layer.

DESCRIPTION OF THE INVENTION

It is therefore the object underlying the invention to provide a metalanode with a cover layer, which cover layer is characterized by a newelectrochemical substance having an extraordinarily high electrochemicalactivity.

According to the invention the metal anode is provided with a coverlayer of TlPd₃ O₄. Although the basis metal of the anode can be anysuitable metal the invention is particularly advantageous where thebasis metal is a valve metal. The preparation of and identification ofthe TlPd₃ O₄ is described in our copending patent application Ser. No.516,996, filed October 10, 1974 now U.S. Pat. No. 3,991,158. Thiscompound is readily soluble in constantly boiling hydrobromic acid (withsome precipitation of TlBr), is moderately soluble in aqua regia andperchloric acid, is difficult to dissolve in sulphuric acid, nitricacid, formic acid and alkaline solvent. Above 750° C., TlPd₃ O₄ isdecomposed while forming elementary palladium and Tl₂ O₃. Thepycnometric density was found to be 8.99 g/cm³. The radiographicevaluation for TlPd₃ O₄ results in a cubic elementary cell:

    ______________________________________                                        lattice constant                                                                          a        =       9.596 ± 0.002 A                               volume of the                                                                 elementary cell                                                                           V.sub.Ez =       883.68 A.sup.3                                   number of                                                                     formula units                                                                             Z        =       8                                                X-ray density                                                                             S.sub.Ro =       5.11 oz./cu.in.                                  ______________________________________                                    

From the crystal lattice plane statistic the following extinctionconditions resulted:

    ______________________________________                                        hK1 :    h + K         =         2n-1                                                  k + 1         =         2n-1                                                  (h + 1)       =         2n-1                                         ______________________________________                                    

On the basis of these extinction conditions the space groups Fm3m-O_(h)⁵, F432-O³, F43m - T_(d) ³, Fm3 T_(h) ³ and F23- T² come intoconsideration for TlPd₃ O₄.

The raster-electromicroscopic examination resulted in the presence ofpartially twinned octahedrones. Viewing the results in combination, theyconfirm the presence of the cubic face-centered structure.

If one now applies the TlPd₃ O₄ to alloyed or unalloyed metal anode basemember alone or together with co-adhesives the electrolysis of NaCl,KCl, chlorate, and HCl solutions can be conducted as well as otherelectrochemical processes.

Below is given an example for the preparation of TlPd₃ O₄ aselectrochemically active substance for electrochemical processes.

EXAMPLE

1 mol PdO is intimately mixed with 4 to 5 mol of TlNO₃ and is slowlyheated to 500° - 600° C in sintered corundum trays. In this connection,by decomposition of the thallium nitrates above 300° C the cubic Tl₂ O₃results in jet-black crystals which react with PdO starting from 500° C.Excess Tl₂ O₃ is removed by sublimation at 650° C; and palladium isremoved by boiling with aqua regia. The cardinal-red reaction productaccording to guinea photos contains no further impurities.

The analyses of this compound is complicated insofar as TlPd₃ O₄ is wellsoluble only in hydrobromic acid. However, the presence of bromide ionsis disturbing in the course of the analyses by the formation of theTlBr, that is difficult to dissolve, and PdBr₂. Thus, it was necessaryto boil the hydrobromic acid solution of the TlPd₃ O₄ with a few dropsof elementary bromine in order to bring the TlBr, which alwaysprecipitates in smaller amounts, in solution as TlBr₃ ; subsequently, toquantitatively expel bromine by boiling with concentrated HNO₃ and,since the oxidizing properties of the nitric acid also adversely affectthe applied precipitation with organic reagents, to fume them off withsulphuric acid. The gravimetric determination of the thallium wasconducted by precipitation with thionalide. Palladium is precipitatedwith dimethylglyoxime.

    ______________________________________                                        TlPd.sub.3 O.sub.4 Test Results                                               Content of Tl    Content of Pd                                                ______________________________________                                        34.8%             54.3%                                                       34.6%             54.8%                                                       34.9%             54.6%                                                       34.9%             54.5%                                                       ______________________________________                                    

Tests have been conducted with different percentages of TlPd₃ O₄ withinthe electrochemically active cover layer. Table 1 is illustrative of thetypical results attained in such tests.

                  Table 1                                                         ______________________________________                                                          Oxygen O.sub.2                                                                within             Cell                                             Cell      gaseous    Hydrogen                                                                              Voltage                                  Percentage                                                                            Voltage   chlorine   H.sub.2 at begin                                 TlPd.sub.3 O.sub.4                                                                    Volt      % volume   % volume                                                                              Volt                                     ______________________________________                                        20      4.10 - 4.27                                                                             0.15 - 0.20                                                                              0.15 - 0.25                                                                           4.10 - 4.15                              40      4.10 - 4.23                                                                             0.15 - 0.20                                                                              0.10 - 0.20                                                                           4.10 - 4.15                              70      4.10 - 4.20                                                                             0.10 - 0.15                                                                              0.10 - 0.20                                                                           4.10 - 4.12                              The tests have been conducted under the following                             standard conditions.                                                          Electrolyte       300 g NaCl/1000 cm.sup.3                                    pH-value          3.0 to 4.0                                                  Temperature       80° C                                                Current density i.sub.A                                                                         10 kA/m.sup.2                                               Distance of the                                                               electrodes        2 mm                                                        ______________________________________                                    

The values indicate that the electrochemical activity of the anode isincreased in direction of a selective separation of chlorine byincreasing the percentages of TlPd₃ O₄ in the anode, and that the agingof the cover layer is decreased so that longer intervals of operation ofthe cells are possible. Because of the high currents used, a lowering ofmaximum cell voltage as little as 0.01 volt is economically significant.Although 20% TlPd₃ O₄ makes a valuable contribution to the coating, itmay be noted that 70% TlPd₃ O₄ lowered the maximum initial voltage 0.03volts and the maximum overall operating voltage 0.07 volts as comparedto 20% TlPd₃ O₄, for example.

                  Table 2                                                         ______________________________________                                        Inlet of the Cell  Outlet of the Cell                                                     g          g                                                      Electrolyte:                                                                              190        125    NaCl/1000 cm.sup.3                              Electrolyte:                                                                              340        565    NaClO.sub.3 /1000 cm.sup.3                      Electrolyte:                                                                              3 - 4      3 - 4  Na.sub.2 Cr.sub.2 O.sub.7 /1000 cm.sup.3        pH-value :  6.0 - 6.5                                                         Temperature:                                                                              60° C                                                      Current density i.sub.A :                                                                 2 - 5 kA/m.sup.2                                                  Content   Cell voltage                                                        TlPd.sub.3 O.sub.4                                                                      characteristic    Current yield                                     %         Volt              %                                                 ______________________________________                                        70        E.sub.Z = 2.50 + 0.31 · i.sub.A                                                        94 - 96                                           50        E.sub.Z = 2.55 + 0.31 · i.sub.A                                                        93 - 95                                           20        E.sub.Z = 2.60 + 0.32 · i.sub.A                                                        94 - 96                                           ______________________________________                                    

A satisfactory electrochemical effectiveness is ensured already when 20%of TlPd₃ O₄ are present in the cover layer. In permanent operation(10.000 A/m²) a thus coated metal anode worked well over a period oftime of 12 months without showing an alteration of the electricalproperties as current yield, cell voltage, or electrochemical activity,such as chlorine deposition and resistance of the cover layer againstwear.

It is, of course, to be understood that the present invention is, by nomeans, limited to the particular Example, but also comprises anymodifications within the scope of the appended claims.

What is claimed is:
 1. In an anode for electrochemical processes whichanode comprises a basis metal and a cover layer, the improvement whereinsaid cover layer comprises cardinal-red cubic face centered thalliumpalladate having the formula TlPd₃ O₄.
 2. The anode as defined in claim1 wherein said cover layer is comprised substantially entirely of saidTlPd₃ O₄.
 3. The anode as defined in claim 1 wherein said cover layercomprises above about 70 percent of said TlPd₃ O₄.
 4. The anode asdefined in claim 1 wherein said cover layer comprises above about 20percent of said TlPd₃ O₄.
 5. The anode as defined in claim 1 whereinsaid basis metal is a valve metal.